Influence of exogenous iron, calcium, protein and common salt on the bioaccessibility of zinc from cereals and legumes.

We have earlier reported the zinc bioaccessibility from cereals and pulses and documented the influence of heat processing, germination and fermentation on the same. In the present study, we have assessed the influence of exogenous iron and calcium equivalent to their supplemental levels on the bioaccessibility of zinc from food grains that generally are the major components of meal in India. Bioaccessibility measurement was made by a procedure involving equilibrium dialysis during simulated gastrointestinal digestion. Exogenous iron equivalent to therapeutic levels (5mg per 10g of cereal-legume combination) significantly reduced the bioaccessibility of zinc from the food grains tested, the percent reduction being 32.4. Exogenous calcium equivalent to therapeutic levels (83mg per 10g of the cereal-legume combination) also significantly reduced (by 27.4%) the bioaccessibility of zinc from the tested food grains. The negative influence of exogenous iron and calcium was similar in both raw and cooked grains. Such negative influences on the bioaccessibility of zinc were however not seen when exogenous iron and calcium were only moderate (up to four times the intrinsic level). A study of the influence of exogenous protein on the bioaccessibility of zinc from food grains revealed that soy protein isolate added at amounts to result in a total protein content of 20% produced contrasting effects on zinc and iron bioaccessibility from cereals - rice and sorghum. While soy protein had a negative effect on iron bioaccessibility from these food grains, the same produced an enhancing effect on zinc bioaccessibility (an increase of 50% and 90% increase) from raw and cooked grain, respectively). Exogenous sodium chloride (at 5% level) potentiated the positive effect of soy protein on zinc bioaccessibility, and effectively countered its negative effect on iron bioaccessibility. The observed negative influence of supplemental iron and calcium on zinc bioaccessibility suggests that zinc supplementation may be necessary in the Indian context, whenever iron and calcium supplements are taken, to compensate for the reduction in zinc bioaccessibility.